Shunting Solved
Discussion
When you look at the Inlet tract, it is clear that the tract lengths are all different lengths and shapes. Although the center ones have longer trumpets (cylinders 3,4,5,6), the tract lengths are shorter than the end ones (cylinder 1,2,7,8). This is because the runners on the end cylinders cross over to feed the opposite bank, whereas the center ones feed the adjacent cylinders.
Additionally the first two runners (cylinders 1,2) run alongside the coolant exit from the head and the back of the thermostat housing. This means that the air entering these two cylinders is heated more than the others, so they actually run very slightly richer than the others due to the less-dense charge. Overall, the different tract lengths tend to spread out the torque curve whilst slightly reducing the peak value.
The options for changing inlet tract length are really rather limited, and there are other factors that dictate the trumpet length that is best used in the circumstances. Adding a spacer between the manifold and the trumpet base will obviously increase tract length (apart from any heat transmission benefits), and slightly increase plenum volume.
The shape of the top of the trumpet is important, and all the information on this topic contained in Mr Vizard's "How to Modify Your Mini" is still absolutely relevant.
However the most critical thing you can control within the plenum is the distance between the end of the trumpet and the underside of the plenum. It is highly beneficial (as a rule of thumb) to have approximately the same distance between the end of the trumpet and the underside of the plenum, as the diameter of the trumpet. This will take advantage of shock-ramming the inlet at particular engine speeds. Obviously the addition of a spacer between the trumpet base and the plenum will change this relationship.
On a dyno test we did at V8D some years ago, the peak torque of a 5.2 Litre motor could be varied by up to 40 lb/ft by this technique. We pulled the plenum of this motor whilst it was at full blast, to check the effect of the AFM and throttle restriction. The motor immediately lost 40 lb/ft. However by placing a flat plate over the top of the trumpets and adjusting the distance, this could be restored and optimised.
This is one major area where removing the trumpets and blending the base will loose out. Although the peak power can go up by 3-4 BHP, a massive chunk of mid-range torque is lost.
However there is another issue here too. Before the engine comes on cam (even with a standard camshaft), there is a large reverse flow of exhaust gas up the inlet tract during the camshaft overlap period (when the inlet and exhaust valves are open at the same time). When the inlet tract is very short, this reverse flow of gas can reach the top of the trumpet and even spill over into the plenum. Once it is in the plenum, it will contaminate the charge drawn by adjacent trumpets.
Since fuel is generaly injected into the inlet port before the inlet valve opens, any fuel will be caught in the backflow up the inlet too. Therefore adjacent cylinders can be contaminated with both fuel and spent exhaust gasses, in uncontrolled quantities.
Obviously this problem becomes worse with wilder camshafts, due to the increased overlap creating increased backflow up the inlet. Clearly the bigger the engine, the greater the volume of spent gas that will be pushed back up the inlet. If the inlet tract is big enough to contain this reverse flow, then the unwelcome symptoms are heavily reduced.
Due to the layout of the trumpets in the plenum, all cylinders will receive different amounts of contamination in this particular area of operation (i.e. before the engine comes on cam). Exhaust gas in the inlet charge tends to slow the flame front, so more ignition advance is required to ensure the most complete burn. The peak burn temperature is also reduced, and this reduces the undesirable production of oxides of Nitrogen (NOx) which is implicated in Acid Rain formation (see http://en.wikipedia.org/wiki/Acid_rain)
Note that this reverse flow characteristic is used by many engine manufacturers to reduce NOx production without the use of specific Exhaust Gas Recirculation (EGR) equipment. NOx reduction is the whole point of EGR, although EGR also has the welcome side effect of making a petrol engine more economical during cruising.
Hence a really short inlet containing little volume, especially on a large motor, will make the cross-contamination problem worse just before the engine comes on cam. This is one of the reasons in general why throttle bodies can help tame wild camshafts when the inlet tract is long enough, because any contamination does not cross to adjacent cylinders.
In summary this is the reason why (regardless of the engine management system in use) on a plenum-equipped engine (i.e. single throttle upstream of the plenum) where the inlet tracts are not long enough to contain the reverse flow, you will get uneven combustion. Usually this shows up to the driver as "shunting".
Back in the days of carburetors, this reverse flow would appear as "stand-off" above the mouths of the carburetors just before the engine came on cam. This is an interesting one to see on the dyno, and it appears as a dome-shaped cone of fuel above the carburetor mouth (usually a trumpet) an is sometimes over 30cm tall.
This process is how carburetors set fire to foam air filters. First they soak them with fuel during stand-off, and it only takes one cough up the inlet to ignite it. Almost all the V8s I have seen equipped with downdraught carburettors and foam air filters over the trumpets catch fire from time to time. It's no problem if you know how to deal with it - full throttle if possible and keep a fire extinguisher in the car.
Additionally the first two runners (cylinders 1,2) run alongside the coolant exit from the head and the back of the thermostat housing. This means that the air entering these two cylinders is heated more than the others, so they actually run very slightly richer than the others due to the less-dense charge. Overall, the different tract lengths tend to spread out the torque curve whilst slightly reducing the peak value.
The options for changing inlet tract length are really rather limited, and there are other factors that dictate the trumpet length that is best used in the circumstances. Adding a spacer between the manifold and the trumpet base will obviously increase tract length (apart from any heat transmission benefits), and slightly increase plenum volume.
The shape of the top of the trumpet is important, and all the information on this topic contained in Mr Vizard's "How to Modify Your Mini" is still absolutely relevant.
However the most critical thing you can control within the plenum is the distance between the end of the trumpet and the underside of the plenum. It is highly beneficial (as a rule of thumb) to have approximately the same distance between the end of the trumpet and the underside of the plenum, as the diameter of the trumpet. This will take advantage of shock-ramming the inlet at particular engine speeds. Obviously the addition of a spacer between the trumpet base and the plenum will change this relationship.
On a dyno test we did at V8D some years ago, the peak torque of a 5.2 Litre motor could be varied by up to 40 lb/ft by this technique. We pulled the plenum of this motor whilst it was at full blast, to check the effect of the AFM and throttle restriction. The motor immediately lost 40 lb/ft. However by placing a flat plate over the top of the trumpets and adjusting the distance, this could be restored and optimised.
This is one major area where removing the trumpets and blending the base will loose out. Although the peak power can go up by 3-4 BHP, a massive chunk of mid-range torque is lost.
However there is another issue here too. Before the engine comes on cam (even with a standard camshaft), there is a large reverse flow of exhaust gas up the inlet tract during the camshaft overlap period (when the inlet and exhaust valves are open at the same time). When the inlet tract is very short, this reverse flow of gas can reach the top of the trumpet and even spill over into the plenum. Once it is in the plenum, it will contaminate the charge drawn by adjacent trumpets.
Since fuel is generaly injected into the inlet port before the inlet valve opens, any fuel will be caught in the backflow up the inlet too. Therefore adjacent cylinders can be contaminated with both fuel and spent exhaust gasses, in uncontrolled quantities.
Obviously this problem becomes worse with wilder camshafts, due to the increased overlap creating increased backflow up the inlet. Clearly the bigger the engine, the greater the volume of spent gas that will be pushed back up the inlet. If the inlet tract is big enough to contain this reverse flow, then the unwelcome symptoms are heavily reduced.
Due to the layout of the trumpets in the plenum, all cylinders will receive different amounts of contamination in this particular area of operation (i.e. before the engine comes on cam). Exhaust gas in the inlet charge tends to slow the flame front, so more ignition advance is required to ensure the most complete burn. The peak burn temperature is also reduced, and this reduces the undesirable production of oxides of Nitrogen (NOx) which is implicated in Acid Rain formation (see http://en.wikipedia.org/wiki/Acid_rain)
Note that this reverse flow characteristic is used by many engine manufacturers to reduce NOx production without the use of specific Exhaust Gas Recirculation (EGR) equipment. NOx reduction is the whole point of EGR, although EGR also has the welcome side effect of making a petrol engine more economical during cruising.
Hence a really short inlet containing little volume, especially on a large motor, will make the cross-contamination problem worse just before the engine comes on cam. This is one of the reasons in general why throttle bodies can help tame wild camshafts when the inlet tract is long enough, because any contamination does not cross to adjacent cylinders.
In summary this is the reason why (regardless of the engine management system in use) on a plenum-equipped engine (i.e. single throttle upstream of the plenum) where the inlet tracts are not long enough to contain the reverse flow, you will get uneven combustion. Usually this shows up to the driver as "shunting".
Back in the days of carburetors, this reverse flow would appear as "stand-off" above the mouths of the carburetors just before the engine came on cam. This is an interesting one to see on the dyno, and it appears as a dome-shaped cone of fuel above the carburetor mouth (usually a trumpet) an is sometimes over 30cm tall.
This process is how carburetors set fire to foam air filters. First they soak them with fuel during stand-off, and it only takes one cough up the inlet to ignite it. Almost all the V8s I have seen equipped with downdraught carburettors and foam air filters over the trumpets catch fire from time to time. It's no problem if you know how to deal with it - full throttle if possible and keep a fire extinguisher in the car.
Edited by Mark Adams on Saturday 21st November 18:28
as for fuel stand off i have witnessed this on many sidedraught and downdraught set/ups you can see this with the eye if you look hard enough 
So what we need then is big one way valves in the top of the trumpets that close off at the point of reversion. Sorted. 
More seriously there must be some subtle differences between engines that shunt really badly and some that show no trace even though the engines appear the same (if that can ever be said in the TVR world.) Also if you can part compensate for the cross contamination of inlet charge with more advance, then a mappable ignition system could help. I guess that would be easy enough to tests by altering the vaccume on the dizzy when its shunting and see the effect.
More seriously there must be some subtle differences between engines that shunt really badly and some that show no trace even though the engines appear the same (if that can ever be said in the TVR world.) Also if you can part compensate for the cross contamination of inlet charge with more advance, then a mappable ignition system could help. I guess that would be easy enough to tests by altering the vaccume on the dizzy when its shunting and see the effect. Thinking about this a bit more, if Im correct (???) with the engine cycle you never get any cross contamination between cyclinders on the same side, but more across the two banks. That then asks the question that the fresh air supply is on one side only, then contamination would be worse on the cylinder bank that is further from the plenum intake as it would part draw fresh air over the trumpets that are now passing out exhaust gas? I wonder if some sort of baffle, or trumpet modifications could be done to discourage trumpet to trumpet breathing?
Try tracing out the routes of the inlet manifold and the firing order, and you may come up to speed with what many of us have been discussing over the years 
. 2 pairs are always too close together just about whatever you do, even if you have 2 plenums you can only achieve 180deg separation?
. 2 pairs are always too close together just about whatever you do, even if you have 2 plenums you can only achieve 180deg separation?Yes they are, its just all a question of extents... The wilder cam, the smaller capacity... just tuning - whats good for the goose is not good for the gander 
We range from paltry 3.9's to wild cam 5.3's... the port shapes we carve out, cams we use, control installed all need to be considered. There is not a single answer to cover every situation..
BUT most of the cars you are considering have not gone that step too far and should be perfectly mannered.. More likely that folks have blindly started trying to tune them and caused it themselves if all the systems are working correctly IMHO.
Unless you can afford throttle bodies and nice sequential firing (and trimming) of individual injectors.... of course
We range from paltry 3.9's to wild cam 5.3's... the port shapes we carve out, cams we use, control installed all need to be considered. There is not a single answer to cover every situation..BUT most of the cars you are considering have not gone that step too far and should be perfectly mannered.. More likely that folks have blindly started trying to tune them and caused it themselves if all the systems are working correctly IMHO.
Unless you can afford throttle bodies and nice sequential firing (and trimming) of individual injectors.... of course
Really intersting stuff !
If it is the case then that the shunting is a problem related to the intake design, am I right in thinking that getting my car's ECU remapped isn't going to help one bit with the shunting issue ?
Also, if each cylinder is contaminating itself anyhow (in the condition where exhaust gases don't spill out of the trumpet), why (in the overspill case) does it matter if a neighbouring one does it too ? Or is it that the overspill then passes into the plenum and there is just a higher percentage of the intake air that is contaminated ? Haven't done a calc to work out how much of the air in the plenum is used per cylinder per stroke.
Matt
If it is the case then that the shunting is a problem related to the intake design, am I right in thinking that getting my car's ECU remapped isn't going to help one bit with the shunting issue ?
Also, if each cylinder is contaminating itself anyhow (in the condition where exhaust gases don't spill out of the trumpet), why (in the overspill case) does it matter if a neighbouring one does it too ? Or is it that the overspill then passes into the plenum and there is just a higher percentage of the intake air that is contaminated ? Haven't done a calc to work out how much of the air in the plenum is used per cylinder per stroke.
Matt
Just to add my experience with my Chimaera '94 4.0L. No cats, TVR none cat map, correct tune resistor gave no shunting. Main cat, TVR cat map and wrong lambdas gave very bad shunting, worse when cold. Main cat, TVR cat map and correct lambdas give some shunting. The TVR none cat EPROM with the cat tune resistor gave very little shunting (I assume this would be a Lucas or Rover base map).
Last year I believe a faulty injector gave the shunting from hell, which caused the valley gasket to go, but that's just what I think caused it and I'm no expert.
Once I can get to the pit in my garage, the cat is coming off again.
Last year I believe a faulty injector gave the shunting from hell, which caused the valley gasket to go, but that's just what I think caused it and I'm no expert.
Once I can get to the pit in my garage, the cat is coming off again.
Just look at all the variables we now have. Basic fuelling, the amounts of ignition advance, air path into the engine, plenum size, plus cam overlap periods & inlet track length causing reversion all thrown into the pot! This is why you will never get a 100% definitive answer or fix. Personally next Id like to try a low level of forced induction just at the shunting point that may be enough just to help keep the plenum clear and get a bit more mixture in. Ill build a test setup and need a volunteer with the problem to test it on!
blitzracing said:
Personally next Id like to try a low level of forced induction just at the shunting point that may be enough just to help keep the plenum clear and get a bit more mixture in.
I thought 'What a Great Idea' then thought a bit more - won't this just effectively increase the air into the induction and make the engine speed increase. The fuelling will again compensate itself via the lambda feedback ?My intial thoughts where the same, but as the problem is so marginal I dont think it would take a big change in plenum / mixture conditions to alter things. Even if it just mixes up the exhaust contamination a bit, or forces a fraction more air in it may be enough. It will help also overcome any pressure drop over the AFM and filter when the car is off cam and breathing poorly. Wont know unless I try it however.
Well I'm back.
As I started all this I thought I better let you all know I have been following with great interest.
Its great to see the subject has sparked so much interest.
The statement that interests me the most was from Mark.
"It is highly beneficial (as a rule of thumb) to have approximately the same distance between the end of the trumpet and the underside of the plenum, as the diameter of the trumpet"
I expect the plenum spacer helped greatly to bring this relationship closer to where it needs to be on my car.
The point of reverse flow of exhaust gas up the inlet tract and the theory that this is worse on an engine that runs a higher lift cam also sounds right to me.
My car is a 4.0HC, the HC gave you a wilder cam and Chimaers thus equipped had a reputation for shunting.
My car was no exception.
However since solving my shunting and creating a much smoother and more pleasurable power delivery I have moved my attentions to improving the fuel economy.
It is clear to me the RV8 does not fuel efficiently between the idle and 2600 rpm range.
As the engine comes on cam everything seems to come together.
At 3000rpm the economy is actually quite respectable for a four litre engine with eight mouths to feed.
I am seeing 28mpg at a steady 80mph in fifth.
I put the poor lower rpm economy down to three things.
1. Induction interference between the trumpets
2. Exhaust gas contamination
3. Rather basic ignition control provided by a distributor using bob weights and bob weight springs
I also think TVR set the fuelling to provide best acceleration off the line.
When driving in the city you are often stopping at traffic lights and then accelerating away, in these conditions I am seeing 13-15mpg.
Although I have my own plans to improve lower rpm fuel efficiency, before I implement them, I welcome your thoughts and ideas.
Thanks all, Dave.
As I started all this I thought I better let you all know I have been following with great interest.
Its great to see the subject has sparked so much interest.
The statement that interests me the most was from Mark.
"It is highly beneficial (as a rule of thumb) to have approximately the same distance between the end of the trumpet and the underside of the plenum, as the diameter of the trumpet"
I expect the plenum spacer helped greatly to bring this relationship closer to where it needs to be on my car.
The point of reverse flow of exhaust gas up the inlet tract and the theory that this is worse on an engine that runs a higher lift cam also sounds right to me.
My car is a 4.0HC, the HC gave you a wilder cam and Chimaers thus equipped had a reputation for shunting.
My car was no exception.
However since solving my shunting and creating a much smoother and more pleasurable power delivery I have moved my attentions to improving the fuel economy.
It is clear to me the RV8 does not fuel efficiently between the idle and 2600 rpm range.
As the engine comes on cam everything seems to come together.
At 3000rpm the economy is actually quite respectable for a four litre engine with eight mouths to feed.
I am seeing 28mpg at a steady 80mph in fifth.
I put the poor lower rpm economy down to three things.
1. Induction interference between the trumpets
2. Exhaust gas contamination
3. Rather basic ignition control provided by a distributor using bob weights and bob weight springs
I also think TVR set the fuelling to provide best acceleration off the line.
When driving in the city you are often stopping at traffic lights and then accelerating away, in these conditions I am seeing 13-15mpg.
Although I have my own plans to improve lower rpm fuel efficiency, before I implement them, I welcome your thoughts and ideas.
Thanks all, Dave.
domV8 said:
STEV8E said:
So in my case it would appear that low speed shunting is caused by the restrictive nature of the precats and the hesitancy at higher speed is cause by the main cat.
blitzracing said:
As for exhaust effecting the shunting, mine disappeared completely when I fitted some extra decibel tubes, but my exhaust is somewhat different to the TVR however with no cat's and straight through side pipes. I assumed the fraction extra back pressure was causing less unburnt mixture to go down the pipes at overlap but may be complete rubbish???
Just to add my 5p and buck the trend - mine has TVR 885 cam with stainless steel manifolds & Y-piece (precats removed and hi-flow 100-cell cat) and straight-through exhaust with no silencers - and it shunts like a b*tch...Dom
Edited by domV8 on Friday 13th November 22:31
Dom
Tvr Power said:
domV8 said:
STEV8E said:
So in my case it would appear that low speed shunting is caused by the restrictive nature of the precats and the hesitancy at higher speed is cause by the main cat.
blitzracing said:
As for exhaust effecting the shunting, mine disappeared completely when I fitted some extra decibel tubes, but my exhaust is somewhat different to the TVR however with no cat's and straight through side pipes. I assumed the fraction extra back pressure was causing less unburnt mixture to go down the pipes at overlap but may be complete rubbish???
Just to add my 5p and buck the trend - mine has TVR 885 cam with stainless steel manifolds & Y-piece (precats removed and hi-flow 100-cell cat) and straight-through exhaust with no silencers - and it shunts like a b*tch...Dom
Edited by domV8 on Friday 13th November 22:31
Dom
I can see that having no cat would reduce the backpressure - but why does having no silencers make a difference..?
Many thanks,
Dom
Looks like this thread came to a sudden halt a month ago, probably with some input from TVRPower. So generically what should I do with my 400 trumpets.....
leave em alone
carbon flared (ACT)
45mm and match/blend the intake manifold (if so what lenght/s)
Should I fit an insulating plate (probably don't need a spacer on a 400)
I have been advised that a 71mm/72mm throttle would achieve no real gains on the 400
thoughts welcome ........ta
leave em alone
carbon flared (ACT)
45mm and match/blend the intake manifold (if so what lenght/s)
Should I fit an insulating plate (probably don't need a spacer on a 400)
I have been advised that a 71mm/72mm throttle would achieve no real gains on the 400
thoughts welcome ........ta
STEV8E said:
I have the spacer and gasket plus smooth bore afm to plenum all ready to go on my 400 in the next couple of weeks. Not expecting much in the power gains dept, but will be interested in the smoothness issue. Just bought the bits for a bit of a tinker really.
As with all the topics will be v interested with your findings.Golflion said:
STEV8E said:
I have the spacer and gasket plus smooth bore afm to plenum all ready to go on my 400 in the next couple of weeks. Not expecting much in the power gains dept, but will be interested in the smoothness issue. Just bought the bits for a bit of a tinker really.
As with all the topics will be v interested with your findings.Gassing Station | Chimaera | Top of Page | What's New | My Stuff